Dom Littlewood goes behind the scenes at some of Britain's most secure institutions to see how they keep us and the country safe. Today, Dom visits Britain's busiest airport, Heathrow, to see how passport control operates. He sees how police horses are trained, then joins mounted police in south Wales as they patrol Cardiff's streets to keep over 60,000 music fans safe at an Ed Sheeran gig. And home security expert Sukie Madahar gives advice on how to protect cars from catalytic converter thieves.